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Abstract

Commercial multi-walled carbon nanotubes (CNT) were functionalized by oxidation with
HNO3, to introduce oxygen-containing surface groups, and by thermal treatments at different
temperatures for their selective removal. The obtained samples were characterized
by adsorption of N2 at -196°C, temperature-programmed desorption and determination of pH at the point
of zero charge. CNT/poly(vinylidene fluoride) composites were prepared using the above
CNT samples, with different filler fractions up to 1 wt%. It was found that oxidation
reduced composite conductivity for a given concentration, shifted the percolation
threshold to higher concentrations, and had no significant effect in the dielectric
response.

Introduction

Carbon nanotubes (CNTs) have attracted particular interest because of their remarkable
mechanical and electrical properties [1]. The combination of these properties with very low densities suggests that CNTs are
ideal candidates for high-performance polymer composites [2]. In order to increase the application range of polymers, highly conductive nanoscale
fillers can be incorporated into the polymeric matrix. As CNTs present high electrical
conductivity (103-104 S/cm), they have been widely used [3]. Therefore, CNT/polymer composites are expected to have several important applications,
namely, in the field of sensors and actuators [4]. However, in order to properly tailor the composite material properties for specific
applications, the relevant conduction mechanisms must be better understood.

The experimental percolation thresholds for CNT composites results in a wide range
of values for the same type of CNT/polymer composites [5], being a deviation from the bounds predicted by the excluded volume theory and a
dispersion for the values of the critical exponent (t) [6,7]. It was demonstrated that the conductivity of CNT/polymer composites can be described
by a single junction expression [8] and that the electrical properties also strongly depend on the characteristics of
the polymer matrix [9]. This article explores the effects of nanotubes surface modifications in the electrical
response of the composites.

Experimental

Preparation and characterization of the modified CNT samples

Commercial multi-walled CNTs (Nanocyl - 3100) have been used as received (sample CNTs).
Further details on this material can be found elsewhere [10]. CNTs sample was functionalized by oxidation under reflux with HNO3 (7 M) for 3 h at 130°C, followed by washing with distilled water until neutral pH,
and drying overnight at 120°C (sample CNTox was obtained). The CNTox material was
heat treated under inert atmosphere (N2) at 400°C for 1 h (sample CNTox400) and at 900°C for 1 h (sample CNTox900), to selectively
remove surface groups. The obtained samples were characterized by adsorption of N2 at -196°C, temperature-programmed desorption (TPD) and determination of pH at the
point of zero charge (pHPZC) from acid-base titration according to the method of the literature [11]. The total amounts of CO and CO2 evolved from the samples were obtained by integration of the TPD spectra.

Composites preparation

Polymer films with thicknesses between 40 and 50 μm were produced by mixing different
amounts of CNT (from 0.1 to 1.0%) with N, N-dimethylformamide (DMF, Merck 99.5%) and PVDF (Solef 1010, supplied by Solvay Inc.,
molecular weight = 352 × 103 g/mol) according to the procedure described previously [9]. Solvent evaporation, and consequent crystallization, was performed inside an oven
at controlled temperature. The samples were crystallized for 60 min at 120°C to ensure
the evaporation of all DMF solvents. After the crystallization process, the samples
were heated until 230°C and maintained at that temperature for 15 min to melt and
erase all polymer memory. This procedure produced α-PVDF crystalline phase samples
[12].

Sample characterization

Topography of the samples and CNT distribution was performed by scanning electron
microscopy (SEM, FEI - NOVA NanoSEM 200). The dielectric response of the nanocomposites
was evaluated by dielectric measurements with a Quadtech 1920. Circular gold electrodes
of 5-mm diameter were evaporated by sputtering onto both sides of each sample. The
complex permittivity was obtained by measuring the capacity and tan δ in the frequency
range of 100 Hz to 100 kHz at room temperature. The volume resistivity of the samples
was obtained by measuring the characteristic I-V curves at room temperature using a Keithley 6487 picoammeter/Voltage source.

Results and discussion

Characterization of CNT samples

Oxidations with HNO3 originate materials with large amounts of surface acidic groups, mainly carboxylic
acids and, to a smaller extent, lactones, anhydrides, and phenol groups [10,13,14]. These oxygenated groups (Figure 1) are formed at the edges/ends and defects of graphitic sheets [15]. The different surface-oxygenated groups created upon oxidizing treatments decompose
by heating, releasing CO and/or CO2, during a TPD experiment. As this release occurs at specific temperatures, identification
of the surface groups is possible [10,13,14]. It is well known that CO2 formation results from the decomposition of carboxylic acids at low temperature, and
lactones at higher temperature; carboxylic anhydrides originate both CO and CO2; phenols and carbonyl/quinone groups produce CO [10,13,14].

Figure 2 shows the TPD spectra of the CNT before and after the different treatments. It is
clear that the treatment with HNO3 produces a large amount of acidic oxygen groups, such as carboxylic acids, anhydrides,
and lactones, which decompose to release CO2. Part of these groups (carboxylic acids) is removed by heating at 400°C. A treatment
at 900°C removes all the groups, so that the obtained sample is similar to the original.
The total amounts of CO and CO2 evolved from the samples, obtained by integration of the TPD spectra, are presented
in Table 1.

Figure 2.TPD spectra of the CNT samples before and after the oxidizing treatments: CO2 (a) and CO (b) evolution.

Table 1. BET surface areas obtained by adsorption of N2 at -196°C and amounts of CO2 and CO obtained by integration of areas under TPD spectra

All the samples release higher amounts of CO than CO2 groups (Table 1). The CNTox sample has the highest amount of surface oxygen. This sample also presents
the lowest ratio CO/CO2 and the lowest value of pHPZC, indicating that this is the most acidic sample. CNTox900 presents the highest CO/CO2 ratio, suggesting the less-acidic characteristics, which matches well with the pHPZC results (Table 1). The acidic character of the samples decreases by increasing the thermal treatment
temperature, since the acidic groups are removed at lower temperatures than neutral
and basic groups, as seen in previous studies [10,13,14].

The CNT samples have N2 adsorption isotherms of type II (not shown), as expected for non-porous materials
[16]. The surface areas of the samples, calculated by the BET method (SBET), are included in Table 1. It can be observed that the oxidation treatments lead to an increase of the specific
surface area. This occurs because the process opens the endcaps of CNTs and creates
sidewall openings [17]. The specific surface areas of the samples slightly increase as the thermal treatment
temperature increases, since carboxylic acids and other groups, introduced during
oxidation, are removed.

Composites processing and characterization

The morphology and fiber distribution of the composite samples were analyzed by SEM
to evaluate the CNT dispersion in the polymeric matrix and determine how the composites
influence the polymer crystallization microstructure. Figure 3 shows the SEM images for the PVDF/CNT composites. The main relevant microstructural
feature of the composite is that the CNT are randomly distributed into the polymeric
matrix. The spherulitic structure characteristic of the pure PVDF is still present
in all the composites samples [12,18].

Figure 3.SEM images for the PVDF@ CNTox400 composites (for 0.2% CNTox400): (a) surface image showing the spherulitic microstructure of the polymer and (b) fracture image showing the dispersion of the CNT into the bulk of the polymeric matrix.

CNT agglomerates are nevertheless more often observed for the CNTox composites samples,
especially for the ones treated at the highest temperatures. With respect to the electrical
properties, oxidation reduces the composite conductivity for a given concentration
and shifts the percolation threshold to higher concentrations (Figure 4). This behavior is mainly due to the reduction of the surface conductivity of the
CNTs due to the oxidation process [8], and is similar for all the functionalized composites. Further, the increase of surface
area due to the functionalization treatment certainly causes surface defects on the
CNTs that also reduced electrical conductivity. The increase of agglomerations for
the treated samples should not have, on the other hand, a large influence in the electrical
response [8]. A change of several orders of magnitude of the electrical resistivity with increasing
CNTs concentration was observed for all samples, indicating a percolative behavior
of the nanocomposites. In general, both in surface (not shown) and in bulk resistivity
(Figure 4a), the percolation threshold appears between 0.2 wt.% for the original CNT samples
and shifts to 0.5 wt.% CNTs for the functionalized nanocomposites.

Figure 4.Electrical response of the PVDF/CNT nanocomposites: (a) Volume resistivity of the PVDF/CNT nanocomposites for the different functionalized
CNTs; (b) dielectric constant at room temperature and 10 kHz for the PVDF/CNT original composites.

Dielectric measurements show that the incorporation of the CNT in the PVDF matrix
but leads to a gradual increase of the dielectric constant (ε') as the amount of the
filler is increased (Figure 4b). The increase of the ε' is larger for the pristine CNT. A maximum for the 0.5% pristine
CNT sample with ε' 22 at a frequency of 10 kHz at room temperature was found, whereas
for the functionalized nanocomposites the value is 16. The frequency behavior of the
dielectric permittivity is similar to the one obtained for the pure polymer, except
for an increase of the low frequency dielectric constant and dielectric loss (not
shown) with increasing CNT loading due to interfacial polarization effects (Figure
4b). No noticeable differences have been observed for the different oxidation treatments
in terms of the dielectric response. In a previous study [19], it was demonstrated that an increase in the dielectric constant is related with
the formation of a capacitor network.

Conclusions

The effect of surface modifications of multi-walled CNTs on the electrical response
of CNT/PVDF nanocomposites has been investigated. The main effect of oxidation is
a reduction of the composite conductivity for a given concentration and a shift of
the percolation threshold to higher concentrations. On the other hand, no significant
differences have been observed between the nanocomposites prepared with the different
functionalized CNTs. The reduction of the electrical surface conductivity of the CNT
due to the oxidation process, together with an increase of the surface area and defect
formation, is at the origin of the observed effects.

Abbreviations

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

SACC performed the functionalisation and characterisation of carbon nanotubes samples
and drafted the manuscript. JNP, CP, and VS participated in the nanocomposite samples
processing, experimental measurements, analysis and interpretation of the results.
MFRP and SL-M conceived and coordinated the research work and carried out analysis
and interpretation of the experimental results. All authors read and approved the
final manuscript.

Acknowledgements

The authors thank the Fundação para a Ciência e a Tecnologia (FCT), Portugal, for
financial support through the projects PTDC/CTM/69316/2006 and NANO/NMed-SD/0156/2007),
and CIENCIA 2007 program for SAC. V.S. and J.N.P. also thank FCT for the SFRH/BPD/63148/2009
and SFRH/BD/66930/2009 grants.